Biobased technologies are quickly u

Biobased technologies are quickly upcoming to take part in the closing of carbon- and other waste streams in our society. However, many of these biobased technologies cannot deal with recalcitrant substrates, such as lignified biomass or municipal waste. Conversion of such carbon-wastes via gasification technology yields synthesis gas (syngas), a mixture of mainly CO, H2 and CO2, giving opportunity to access the full carbon spectrum of the initial material via gas fermentation. Alternative sources interesting for gas fermentation are offgases from industry (e.g., steel mills) and syngas generated via high temperature coelectrolysis (HTCE), reforming steam and CO2 into syngas.1 HTCE can also be operated with solely sunlight as energy source, deriving syngas from inorganic sources.2 Making use of microbial gas fermentation processes, waste streams can be converted via a uniform substrate into fuels and commodity chemicals. Companies, such as Lanzatech, establish large scale production systems to generate biobased products from CO-rich, steel mill waste gases, showing the application potential of gas fermentation technology.3,4Carbon monoxide is one of the main components in untreated syngas, and is known as an odorless, colorless and toxic gas. Despite its toxicity, it can act as a natural substrate for anaerobic microorganisms, driving acetogenic, hydrogenogenic and methanogenic metabolisms.5 Its low reduction potential (E0′ = −520 mV) makes it a strong electron donor, and theoretically allows for higher energy conservation compared to hydrogen oxidation (E0′ = −414 mV). However, generally methanogens grow poorly on CO. This can also be deduced from the fact that only four methanogens have been shown to grow on CO as a sole substrate: Methanosarcina acetivorans,6 Methanothermobacter thermoautotrophicus,7 Methanothermobacter marburgensis8 and Methanosarcina barkeri.7 The hydrogenotrophic methanogens M. thermoautotrophicus and M. marburgensis both showed CO conversion to methane but preferred H2/CO2 over CO.